Anatomy of the Human Eye
Rocky Young, Ph.D.
Professor of Ophthalmology & Visual Sciences
- General organization
- Layers of the eyeball
- Outer layer: Cornea (1/6, transparent) & Sclera (5/6, opaque)
- Middle layer (Uveal tract): Choroid, Ciliary body, & Iris
- Inner layer: Retinal pigment epithelium & sensory retina
- Compartments of the eyeball
- Aqueous compartment (anterior and posterior chambers)
- Vitreous compartment
- Exterior anatomy
- 1.Conjunctiva -- vascular lining on the anterior surface of eyeball (excluding the
- 2.Anterior ciliary arteries enter eye just behind corneosclera border
- 3.Vortex veins exit just behind of equator of the eyeball
- 4.Extraocular muscles attached to eyeball: 4 recti & 2 obliques
- 5.Ciliary nerves and posterior ciliary arteries (enter eye around optic nerve)
- 6.Optic nerve (emerges from nasal side and below posterior pole)
- 7.Central retinal artery & vein (enter eyeball with optic nerve)
- Outer layer of the eye
- Cornea is the major (optical) refractive element of the eye.
- Corneal curvature & index of refraction account for optical power.
- Transparency attributed to no blood vessels, relatively few cells, & the arrangement
of collagen fibers in mucopolysaccharide matrix.
- Main layers: Epithelium (5-6 cells thick, provides barrier between external and
intraocular environment), stroma (90% of thickness, composed of collagen fibers),
endothelium (maintains relative dehydration of corneal stroma).
- Tear film layer produces "optical smoothness": Oily layer (from meibomian
gland), aqueous layer (from lacrimal gland), & mucoid layer (from goblet cells).
- Cornea has pain reception (ciliary n).
- Oxygen & nutrition: Air & tears (anteriorly), aqueous (posteriorly), and limbal
circulation (around circumference).
- Sclera is the "white" of the eye.
- Sclera itself is dense, fibrous tissue. Like cornea, it is composed of collagen fibers.
Water content, arrangement of diameter of collagen fibers, and type of mucopolysaccharides
all contribute to opacity.
- Sclera is covered by other tissues, e.g., conjunctiva &
- Has pain reception (ciliary n)
- Lamina cribosa = sieve-like structure in the sclera through which optic nerve fibers
exit the eyeball.
- Limbus = the transitional region from cornea and sclera. Located here are:
- Blood vessels and lymphatics.
- Trabecular network and canal of Schlemm, involved in the drainage of the aqueous humor.
- Middle layer of the eye ( "uveal tract")
- Iris = a diaphragm whose aperture (the pupil) changes size.
- Two layers, stroma and the epithelium.
- Stroma consists of connective tissue, blood vessels, & sphincter muscle. Amount of
melanin here determines the color of the iris. If only slight amounts, light reflections
from the pigmented epithelium causes scattering and thus a blue color.
- Epithelium: (i) Myoepithelium consists of dilator muscle. (ii) Pigment layer has cells
packed with melanin.
- Sphincter muscle is a 1-mm band of smooth muscle that encircles the pupil. When it
contracts, the pupil constricts. N III (parasympathetic) via ciliary ganglion to short
- Dilator muscle runs radially. When it contracts, pupil dilates. Sympathetic N via
ciliary ganglion to ciliary n.
- The iris separates the anterior and posterior chambers of the aqueous compartment.
- Ciliary body = ring of tissue located between the iris and choroid. Ciliary body is
divided into epithelial (inner) and uveal (outer) portions.
- Epithelial portion produces aqueous humor. Two parts of the epithelial portion are the
pars plicata ("folds") and the pars plana ("featureless").
- Pars plicata has some 60-70 folds (the "ciliary processes").
- Pars plana is the portion closest to
- Uveal portion contains vascular supply and ciliary muscles (involved in accommodation).
Contraction of the muscles makes the lens more convex (more optical power).
- Zonules (zonules of Zinn) = fibers holding the lens to the ciliary body.
- Ora ("border") serrata = border between ciliary body and choroid. Retina
abruptly ends here. Has serrated margin.
- Choroid =posterior portion of the uveal tract; is a highly vascular tissue.
- Suprachoroid layer --- loosely attaches choroid to sclera.
- Layers of blood vessels:
- Large vessels (Haller's layer): mostly veins, empties into vortex v.
- Medium vessels (Sattler's layer): mostly veins, empties into vortex v.
- Choriocapillaris: Supplies retinal pigment epithelium & outer retina.
- Bruch's membrane layer -- transition between choriocapillaris & retinal pigment
- Macroscopic appearance is black due to pigmentation.
- Choroid also contains nerves and arteries to anterior portion of eye.
- Inner layer of the eye
- Retinal pigment epithelium: Single layer of (pigmented) cells supporting metabolic
activity of photoreceptors. Microvilli at the apical surface "embrace"
photoreceptor outer segments. Remember choriocapillaris at the basal surface of
- Sensory retina --- multiple layers:
- Three layers of cells: Photoreceptor cell, inner nuclear (horizontal, bipolar &
amacrine cells), & ganglion cell layers.
- Two synaptic layers: Outer plexiform and inner
- Skeletal support: Mueller cells (traversing retinal thickness) & glia cells (nerve
- Other layers: Nerve fiber layer, inner & outer limiting membranes.
- Ophthalmoscopic landmarks: Fovea, macula, optic disk, nerve fiber layer, ora serrata,
retinal vascular arcade, macular pigment.
- Optical transparency of inner retinal layers: Pigmentation, retinal blood vessels, and
lack of myelination.
- Aqueous humor maintains the intraocular pressure in the eye. Supplies oxygen &
metabolites to (as well as removes waste from) avascular structures.
- Aqueous is secreted by "ciliary processs" (see pars
- It flows from the posterior chamber through the pupil into the anterior chamber.
- It drains through the trabecular network, passes through the canal of Schlemm, and exits
into the episcleral veins.
- Lens is a transparent biconvex body which provides variable optical power (i.e., allows
the eye to focus at different distances).
- Anatomy: Lies behind the iris. Has capsule, cortex, and nucleus.
- Attachment in eye: Capsule attached to the ciliary body by zonules (zonule of
- Accommodation: Contraction of ciliary muscles (parasympath III n) leads to increase in
- Aging effect: New fibers are formed under the capsule around the circumference and
pushed to the center of lens. Zones or bands of discontinuity in the cross section of a
lens delineate tissue age.
- Vitreous body is a transparent, viscous fluid or gel-like structure composed of a
network of collagen fibers suspended in a liquid containing hyaluronic acid.
- Vitreous is of no importance in maintaining the shape of the eye, but the physical
relationship between the vitreous gel and the retina is the basis of serious eye problems.
- Points of attachment in the eye: (1) Annular zone of adhesion straddling the ora
serrata, (2) adhesion between gel and posterior lens capsule, (3) adhesion to the retina
via the "inner limiting membrane".